1. Field of the Invention
The invention relates to a cadmium/rare gas discharge lamp of the short arc type in which cadmium and rare gas contribute to a discharge emission. The invention relates in particular to a cadmium/rare gas discharge lamp of the short arc type that is suited for light sources of optical devices in which ultraviolet rays are used.
The invention further relates to a cadmium/rare gas discharge lamp of the short arc type that emits ion lines in a C-range with high energy exchange efficiency, and the ion lines are emitted from ultraviolet rays of cadmium ions.
The invention further relates to a cadmium metal vapor discharge lamp in which an emission of cadmium ions is used. The invention relates in particular to a cadmium metal vapor discharge lamp that is suited for light sources of optical devices in which ultraviolet rays are used.
2. Description of Related Art
As is known, optical devices in which ultraviolet rays are used are widely used for industrial applications such as for reforming plastic surfaces, for photo chemical vapor deposition (CVD), for photo-incineration, for UV curing in which a certain wavelength is needed, for photolithography and for similar purposes.
When using light in a wavelength of 185 nm to 300 nm, it is usually desired to use a metal/rare gas discharge lamp such as a xenon-mercury lamp or a xenon cadmium lamp. It is purported that, when using light in particular in a wavelength of 220.+-.20 nm, a xenon/cadmium discharge lamp is suitable, as, for example, it can be concluded from Japanese laidopen specification SHO 55-10757 entitled "Cadmium/rare Gas Discharge Lamp of the Short Arc Type."
In Japanese laid-open specification SHO 55-10757, it is pointed out that, by determining the encapsulation amount of cadmium and rare gas, radiation in a wavelength around 220 nm can be intensified, which can shorten the heat curing time in a production operation of a semiconductor device.
Emission spectra of a cadmium lamp in a wavelength range of 210 nm to 230 nm are achieved because of a subtle balance of the density of the distribution number of cadmium atoms, ions and molecules that are in a ground state. To achieve a needed form of the spectra it is therefore necessary to achieve a suitable density and a suitable vapor pressure by controlling an encapsulation amount of cadmium.
The density of the amount of cadmium and of cadmium vapor pressure inside a discharge space are, on the other hand, very strongly influenced by the temperature of the coolest part of the arc tube in a lighting operation. As a result of this, the temperature of the coolest part also exerts a strong influence on the distribution of emission spectra.
The propagation of band spectra of Cd.sub.2 that contain, because of a certain encapsulation amount of cadmium, fine spectra of monovalent Cd ions with a wavelength of 214.4 nm, reacts, for example, very sensitively on the vapor pressure of the cadmium. To intensify the radiation wavelength in a wavelength range around about 220 nm, it is therefore necessary, in a cadmium lamp, to stabilize the intensity of the band spectra including 214.4 nm by performing any thermal insulation or any temperature regulation of the tube wall, so that a sufficient vapor pressure of the cadmium can be achieved.
But, in a lamp in which a thermal insulation or any temperature regulation of the tube wall is performed, in this way, to keep the vapor pressure under control, it is considered a drawback that the lamp voltage fluctuates after a short period.
In connection with a cadmium/rare gas discharge lamp of the short arc type, the following is to be noted.
It is known that inert gas that is encapsulated in a cadmium vapor discharge lamp, which hereafter is referred to only as a cadmium lamp, has two functions, namely, a thermal insulating effect to achieve a metal vapor pressure required for emission within a lamp bulb or a simplification of a transition from a glow discharge to an arc discharge, i.e., an improved startup characteristic.
The above-described circumstances do not exclude a cadmium lamp usually used in practice either, in which Xe gas with slight thermal conductivity is often used as the inert gas.
As can be concluded, for example, from the Japanese laid-open specification SHO 55-10757 entitled "Cadmium/rare Gas Discharge Lamp of the Short Arc Type" or German patent 16 39 112 entitled "Metal Vapor Discharge Lamp for Photochemical Purposes," Xe gas is described as a suitable inert gas in embodiments.
But, the present inventors have found out, by experiments in which Xe gas was selected as the inert gas and a relationship was investigated between an encapsulation pressure of the Xe gas and a light efficiency of light radiated as ultraviolet rays in a wavelength range of 200 nm to 250 nm, that the light efficiency is reduced as a function of an increase in the encapsulation pressure of the Xe gas.
A light efficiency of ultraviolet rays in a C-range in a cadmium lamp, i.e., in a wavelength range of less than or equal to 250 nm, can also regulated by cadmium vapor pressure and by lamp current. That means that the light efficiency, as a result of a self-absorption of a resonance line with 228.8 nm of a neutral cadmium, goes down when, to achieve high light efficiency, the cadmium vapor pressure is excessively raised. If, on the other hand, the cadmium vapor pressure is too low, a density of a distribution number in an excitation state connected with emission, and thus the light efficiency, go down.
To increase light efficiency at a suitable cadmium vapor pressure by excitation with high efficiency in an excitation state connected with emission, it is generally known, frequently, to use inert gas which, to increase gas temperature of an arc, exhibits an ionization potential that is higher than the ionization potential of an emission substance.
Starting from the above-described circumstances, Xe gas is used in a cadmium lamp, Xe gas that is encapsulated in a relatively simple way during a production operation of the lamp. Since an ionization potential of cadmium atoms is 8.99 eV and an ionization potential of Xe atoms is 12.13 eV, the above-described condition-is thus fulfilled.
But, by experiments of the present inventors, it was determined that, by increasing the encapsulation pressure of the Xe gas to increase the gas temperature of the arc, the light efficiency of the cadmium lamp in the wavelength range of 200 nm to 250 nm goes down, because input energy is not used to excite the cadmium but, as a result of the excitation of the Xe gas, a large amount of energy is used. Therefore, it was difficult to increase the light efficiency of the cadmium in the wavelength range of 200 nm to 250 nm.
It is generally known for a cadmium metal vapor discharge lamp (hereafter referred to only as a cadmium lamp) that, when using cadmium as the main emission substance, it uses line spectra of neutral cadmium, for example, a radiation wavelength of 228.8 nm or the like.
Recently wavelengths of light that is used for industrial applications are becoming increasingly shorter, corresponding to the requirements in the development of photochemical industries, production fields of semiconductor devices or the like.
But, in a cadmium lamp using a resonance line with an emission wavelength of 228.8 nm from cadmium, as a result of a self-absorption phenomenon, it can happen that insufficient light intensity is achieved if, to achieve a strong light from cadmium, cadmium partial pressure is increased during operation. If, on the other hand, to avoid the self-absorption phenomenon, the partial pressure is reduced during operation, it can happen that, as a result of reduced emission substance, a light intensity sufficient for industrial applications is not achieved either.
To eliminate the above-described drawback, a cadmium lamp is investigated that uses emission from cadmium ions. In this cadmium lamp, a considerably higher density of cadmium ions is used than in a conventional cadmium lamp.
But, a cadmium lamp using cadmium ions that radiates shortwave light at an intensity sufficient for industrial applications has the same drawback of such a cadmium lamp that is caused by the fact that it is necessary to produce, within a tube, a density of cadmium ions that is unusually high for a conventional cadmium lamp. This drawback consists in that, during a relatively short lighting period, a cloudiness, i.e., a so-called devitrification phenomenon, occurs on an inner side of the tube and in that, as a result of this, after a relatively short period of use of the lamp, sufficient light cannot be obtained any more.